1. Field of the Invention
This invention relates to methods for utilizing geothermal resources to produce energy, and, in particular, to methods of maintaining the volume of the liquid injected back into the ground.
2. State of the Art
Geothermal fluids contain vast amounts of thermal energy, which can be used to produce electrical power. In one process for producing electrical power, a naturally pressurized, hot, substantially liquid geothermal brine is recovered from a subterranean reservoir, then suddenly depressurized, or, as termed herein, flashed. This produces steam from the brine while cooling the liquid brine. The cooled geothermal brine is processed further and then typically injected into the ground to replenish the aquifer and prevent ground subsidence, particularly in the Salton Sea, Calif., which is already below sea level.
In a geothermal operation, it is important to inject about the same amount of liquid back into the ground as is removed. If water is not added to the subterranean reservoir, then the concentration of solids in the brine removed from the ground will increase. Adding water keeps the concentrations of salts and other solids in the brine removed for flashing at about the same concentration over the period of time the geothermal field is in operation. Liquid cannot just be dumped into the brine liquid stream remaining after flashing to bring it up to the original volume since the brine is extremely sensitive to additions of foreign liquids, and the overall system prone to upsets, if the liquids are not matched exactly correctly.
As the brine is flashed, many of these components (both ions and minerals) precipitate out of the remaining liquid solution. Some minerals, typically silicon-containing compounds--particularly silica and silicate-containing minerals, such as iron silicate--form small particles that precipitate slowly. The slow precipitation means that these already precipitating minerals form solids that plug the downstream fluid handling and injection apparatus. Therefore, it is greatly preferred to force these slowly precipitating solids out of the liquid before injection using a flocculent added to the brine in a clarification process. U.S. Pat. No. 4,874,529, issued to Featherstone, which Patent is hereby incorporated in full by reference, discusses the problem of slowly precipitating fine particulates and describes the solution of adding a flocculating agent to the secondary clarification zone as the solution to the problem.
However, not all minerals precipitate from the brine solution slowly. Alkaline earth salts (for example, barium sulfate, also known as barite, and calcium fluoride) are among the minerals frequently found in significant concentrations in geothermal brines. Alkaline earth salts tend to precipitate fairly quickly. During the flashing step alkaline earth sulfates (for example, barium sulfate) start to precipitate and continue precipitating down-stream during further handling of the brine as it continues to cool. Barium sulfate presents an annoying problem in many geothermal systems since it starts to precipitate when the cooling brine is still as hot as 340.degree. F., a temperature produced early on in the energy production process, triggering the early precipitation of barium sulfate.
Now, the precipitation of barium sulfate has been implicated in a more serious precipitation problem. It has been noticed that the crystalline material precipitated at some geothermal sites becomes more radioactive as the site ages. The radioactivity, contained in naturally occurring radioactive materials, or NORMs, is believed to result from tapping into new subterranean formations containing trace amounts of a radioactive alkaline earth sulfate--radium sulfate--which apparently coprecipitates with the chemically similar barium sulfate. It is important to reduce the levels of NORMs in the material that precipitates during the clarification process on the processing equipment to as low a level as possible. The problem is solved in U.S. patent application Ser. No. 717,527, filed Jun. 19, 1991, now allowed, and incorporated herein in full by reference by using crystal structure modifiers to prevent NORMs precipitation before injection.
Before the brine stream can be injected, the amount of liquid in the brine stream should be brought back to nearly the same amount as was removed from the ground. However, the brines contain dissolved components that often cause processing problems. If surface water is added to the brine stream as it is--without some sort of prior processing--the added water will cause system upsets. It would be advantageous to have a system that allowed the addition of a liquid to bring the volume of the brine back to the removal volume before injection while avoiding system upsets to keep NORMs and the like in solution.